Atomic History Timeline 1942-1944
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Atomic History Timeline 1942-1944

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Chronology For The Origin Of Atomic Weapons

Courtesy of Carey Sublette

I have included here a chronological listing of events and milestones leading up to the use of atomic weapons against Japan. Brief explanatory notes are inserted to provide some context and interpretation. The interested reader is directed to several excellent books available (see bibliography), particularly the Pulitzer Prize winning book by Rhodes, and Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years 1943-1945, from whom the bulk of the material for this timeline was extracted.

The timeline is divided into several epochs which seem to me to be naturally separated by critical events. Each epoch begins with a short summary of the key themes that characterize it. Although this is a strict chronology which list events that are more or less datable, occasional paragraphs are interspersed summarizing the thrust of events. 

The Manhattan Project - The Work Begins in Earnest

*** From September 1942 To January 1945 ***

This phase is the crash program that continues through the end of the war with Japan, and leads to successful development of atomic bombs. Under the aggressive and savvy leadership of Brig. Gen. Groves the program shifts into high gear, and overtakes all other programs in priority. Virtually unlimited money is made available, the only real limitations are how quickly the program can absorb funds and find qualified personnel. Very soon after taking over three methods of producing fissionable material are chosen for full scale development: plutonium production in uranium-graphite reactors, and uranium enrichment using gaseous diffusion and electromagnetic separation. I have separated the early phase of the Manhattan Project, where immense scientific and technical problems had to be overcome to develop feasible designs and production methods, from the later phase where firm designs were translated into practical hardware and actual combat use was planned and executed.

August, 1942 - Col. Marshall of the Army Corps of Engineers creates a new District organization with the intentionally misleading name "Manhattan Engineer District" (MED).

August 29, 1942 - A status report by Conant is relayed to the Secretary of War by Bush indicating the very positive results of Oppenheimer's group. Bush adds his concerns about the organization and leadership of the project, requesting new leadership be appointed.

September 13, 1942 - A meeting of the S-1 Executive Committee discusses the need for a central fast neutron laboratory, to be code-named Project Y.

September 15, 1942 - Starting on this date, and continuing until November 15, Fermi's group receives shipments of uranium and graphite for CP-1 and prepares them for assembly.

September 17, 1942 - Col. Leslie Richard Groves is notified at 10:30 a.m. by Gen. Brehon Somervell that his assignment overseas has been cancelled and that he will take another assignment - command of the Manhattan Engineer District. Groves' previous assignment had required overseeing ten billion dollars worth of construction projects, including the construction of the Pentagon.

September 18, 1942 - Groves buys 1250 tons of high quality Belgian Congo uranium ore stored on Staten Island.

September 19, 1942 - Groves buys Site X, 52000 acres of land on the Clinch River in Tennessee, the future site of Oak Ridge. Preliminary construction work begins soon after.

September 23, 1942 - Groves is promoted to Brigadier General.

September 26, 1942 - At Groves' insistence the Manhattan Project is granted approval by the War Production Board to use the highest emergency procurement priority in existence (AAA) when needed.

September 29, 1942 - Oppenheimer proposes that a "fast-neutron lab" to study fast neutron physics and develop designs for an atomic bomb be created. The idea at this point is for the lab to be a small research institution, it would not be involved in the engineering and production of nuclear weapons.

October, 1942 -

  • Groves puts Du Pont in charge of the plutonium production project.
  • Conant recommends to Bush that information exchange with Britain, already largely one-way (UK -> US), be sharply restricted. Bush passes this recommendation to Roosevelt. As a result the US loses access to British work in gaseous diffusion, which seriously delays successful plant completion.
  • Centrifuge separation is abandoned due to technical problems.

October 5, 1942 - Groves visits the Met Lab and meets the key scientists, including Oppenheimer. He orders key engineering decisions for plutonium production, under debate for months, be made in 5 days.

October 15, 1942 - Groves asks Oppenheimer to head Project Y, planned to be the new central laboratory for weapon physics research and design.

October 19, 1942 - Vannevar Bush approves Oppenheimer's appointment in meeting with Oppenheimer and Groves.

November 3, 1942 - Seaborg reports that due to plutonium's high alpha activity, slight amounts of light element impurities can cause a serious problem with neutron emission from alpha -> n reactions. This issue caused major concern with many project leaders, including Groves and Conant, not only due to its own significance, but because it raised apprehension about the impact of other unexplored phenomena. (This issue later became moot due to the problems with Pu-240 contamination.) Later in the month the Lewis Committee is formed to review progress and make recommendations.

November 16, 1942 -

  • Fermi's group begins constructing CP-1 at Staggs Field using round-the-clock shifts.
  • Groves and Oppenheimer visit the Los Alamos mesa in New Mexico and select it for "Site Y".

December, 1942 -

  • During this month the work on gaseous diffusion is reorganized. On the strength of the Lewis Committee's recommendation, gaseous diffusion is chosen as the principal enrichment approach. Kellex, a subsidiary of Kellog is created to build a plant, Keith is put in charge. Contracts are put in place, and hiring begins for plant construction. Kellex immediately begins work on a process for producing usable barrier material on an industrial scale.
  • Bush provides Roosevelt with an estimate placing the total cost for the Manhattan Project at $400 million (almost 5 times the previous estimate). Roosevelt approves the expenditure.
  • Plans and contracts are made for the construction of an experimental reactor, plutonium separation plant, and electromagnetic separation facility at Oak Ridge.

December 1, 1942 - After 17 days of work, Fermi's group completes CP-1. It contains 36.6 metric tons of uranium oxide, 5.6 metric tons of uranium metal, and 350 metric tons of graphite. Construction is halted sooner than planned when Fermi projects that a critical configuration has been reached.

December 2, 1942 - 3:49 p.m. CP-1 goes critical. It demonstrates a k value of 1.0006, and is allowed to reach a thermal output of 0.5 watts (ultimately it operates at 200 watts maximum)

December 6, 1943 - M. M. Sundt Company is appointed contractor to build Los Alamos Laboratory in a handshake deal. Sundt begins construction immediately, without plans or blueprints in order to finish as quickly as possible.

January, 1943 - Groves acquires the Hanford Engineer Works, 780 square miles of land on the Columbia River in Washington for plutonium production reactors and separation plants.

February 18, 1943 - Construction begins at Oak Ridge on buildings for Y-12, the electromagnetic U-235 separation plant.

March, 1943 - The original construction program nears completion, and staff begins arriving at Los Alamos to begin operations. From this point on the site grows non-stop through the end of the war.

March 27, 1943 - Tolman writes Oppenheimer about using explosives to collapse a shell into a critical mass. This is the earliest surviving reference to the idea of implosion (although this term was not used).

April, 1943 - At the beginning of the month the original building plan for Los Alamos is 96% complete. It is already apparent that the original construction program is inadequate to meet needs.

A series of staff conferences among the ~100 scientific staff members are held at Los Alamos. These include indoctrination lectures by Robert Serber (later published as _The Los Alamos Primer_) on April 5,7,9,12, and 14; and meetings to plan the laboratory's work from April 15 through May 6. The laboratory's initial organization and leadership is worked out.

  • Seth Neddermeyer begins research on implosion, seeking to compress hollow metal assemblies.
  • Bethe is selected over Teller to head the theoretical division. Teller is soon placed in charge of lower priority research on fusion weapons.

Oppenheimer projects that 100 g of 25% enriched U-235 will be produced by electromagnetic separation by 1 Jan. 1944.

From the outset the basic plan for developing nuclear weapons at Los Alamos was to use gun assembly for both uranium and plutonium bombs. This method was well understood from an engineering perspective, and was believed to have a high probability of success. Due to the deadline set by Groves, to have weapons ready to use by summer 1945 (some 26 months away), two important and unusual features for necessary for the program at Los Alamos.

First, the traditional division of scientific/industrial work into research, design engineering, and production engineering were impossible. They had to be conducted concurrently, with overlapping responsibilities and duties. Research had to be conducted specifically to produce reliable, manufacturable designs as quickly as possible. Scientific research that did not directly contribute to this could not be pursued.

Second, the program had to be redundant. All (or several of the most) promising avenues had to pursued simultaneously for nearly every aspect of research and development. Unexpected roadblocks could not be permitted to delay the delivery of a usable weapon. The decision to pursue the rather speculative and initially unpromising implosion idea in addition to the gun technique is an example of this of later major significance.

By the end of the March planning sessions, the necessity of including ordnance development activity at Los Alamos was apparent. This greatly expanded the scope of work undertaken at the laboratory to engineering development, and eventually acting as prime contractor for weapon production, and manufacturer of key weapon components (including all nuclear components, and the implosion system). 

April 1, 1943 -

  • Fencing of the reservation completed, Oak Ridge is closed off to public access.
  • Construction begins on plant for manufacturing gaseous diffusion barriers in Decatur, Ill. although no barrier materials of usable quality have yet been produced.

April 20, 1943 - A contract is concluded with the University of California to manage Los Alamos, acting as paymaster, accountant, and procurement agency. This contract (back dated to Jan. 1 for work already performed) is still in existence and serves as the basis for University of California management of both the Los Alamos and Lawrence Livermore laboratories.

May 10, 1943 - The Los Alamos review committee approves the laboratory's research program.

May 31, 1943 - Surveying begins for K-25, the gaseous diffusion uranium enrichment plant at Oak Ridge.

June, 1943 - Navy Capt. William Parsons arrives at Los Alamos as Ordnance Division leader to begin directing gun assembly research.

June 24, 1943 - Working with cyclotron produced plutonium, Emilio Segre determines that the spontaneous fission rate is 5 fissions/kg-sec. This is well within the assembly speed capability of a high speed gun.

July 4, 1943 - Neddermeyer conducts first explosion in the implosion research program (currently consisting of Neddermeyer, and 3 informal assistants).

July 10-15, 1943 - The first nuclear physics experiment is conducted at Los Alamos (the measurement of Pu-239 fission neutron yield), inaugurating it as a functioning laboratory.

August, 1943 -

  • Despite the efforts of more than 1000 researchers at Kellex and Columbia University, no suitable diffusion barrier material has yet been developed.
  • Due to lagging progress on gaseous diffusion, and continuing uncertainties about the required amount of U-235 for a bomb, Groves decides to double the size of the Y-12 plant.
  • The first Alpha electromagnetic uranium separation unit begins operation. Construction staff at Oak Ridge now exceeds 20,000.
  • Construction begins on the cooling systems for the production reactors at Hanford. Construction staff is about 5,000.

September 17, 1943 - First shot fired in gun assembly research program at Los Alamos. The focus at this point is on developing a high velocity gun for plutonium since a uranium gun would be much easier to make.

September 20, 1943 - Johann Von Neumann arrives on a visit to Los Alamos and points out the potential for high compression from implosion. This is a clear advantage for the technique which would make a bomb more efficient, and require a smaller critical mass. Teller and Bethe begin investigating the subject theoretically, Oppenheimer and Groves become very interested in its potential, and efforts to accelerate the program begin. John Von Neumann agrees to work on the physics of implosion in his spare time.

September 23, 1943 - Oppenheimer suggests recruiting George Kistiakowsky, the leading explosives research director at OSRD, to aid an expanded implosion effort.

October, 1943 -

  • The first Alpha racetrack (containing 96 units) is completed. A work force of 4800 to run Y-12 has been assembled. Start up is unsuccessful due to unexplained shorts in the magnets.
  • Project Alberta, the full scale atomic bomb delivery program, begins. Norman Ramsey appointed to select and modify aircraft for delivering atomic bombs.

October 4, 1943 - Du Pont engineers release reactor design drawings for the first Hanford plutonium production pile, B-100, allowing construction to begin.

October 10, 1943 - Site preparation starts for the B-100 plutonium production reactor at Hanford.

October 21, 1943 - First concrete is poured for the K-25 building at Oak Ridge.

November, 1943 -

  • The top experts in England on fission weapons, many former members of the MAUD committee, depart England for the US to assist the atomic bomb project. Included are Bohr, Frisch, Peierls, Chadwick, William Penney, George Placzek, P.B. Moon, James Tuck, Egon Bretscher, and Klaus Fuchs.
  • The Navy approves Abelson's plan to build a liquid thermal diffusion pilot plant for enriching uranium.
  • The world's first sample of plutonium in metal form is produced by reducing PuF4 with Ba at the Met Lab.

November 4, 1943 -

  • The X-10 pile goes critical at Oak Ridge. This air-cooled experimental pile begins producing the first substantial (gram) amounts of plutonium to assist research into its properties. The world supply of plutonium at this time is 2.5 mg, produced by cyclotrons.
  • A Manhattan Project Governing Board meeting approves an ambitious implosion research program, intended to develop it to the point of usability in six months.

November 29, 1943 - The first B-29 modifications begin at Wright Field, Ohio to adapt it for carrying atomic bombs.

December, 1943 - After attempts to bring the first Alpha racetrack into operation fail, Y-12 is shut down for equipment rebuilding.

  • Segre measures the spontaneous fission rate of U-235 at Los Alamos, and finds it lower than expected. This allows a substantial reduction in performance of the planned gun assembly method for uranium.
  • Chemical separation of reactor-produced plutonium begins, using fuel from the X-10 pile.

January, 1944 -

  • Kistiakowsky arrives at Los Alamos to assist Neddermeyer in implosion research. It becomes increasingly clear that Neddermeyer's academic research style is unsuited to directing a rapidly expanding research and engineering program.
  • Problems with developing suitable diffusion barriers leads Groves to switch planned production to a new type of barrier, creating months of delays in equipping K-25 for operation.
  • Abelson, at the Naval Research Laboratory, begins constructing a thermal diffusion uranium enrichment plant. Upon learning about the problems with the Manhattan Project's gaseous diffusion plant, he leaks information about his technology to Oppenheimer.
  • Groves and Oppenheimer decide to plan for a fission bomb test (none was envisioned before this). Groves stipulates that the active material must be recoverable if a fizzle occurs, so the construction of Jumbo, a 214 ton steel container (25 ft x 12 ft), is authorized.

January 11, 1944 - An implosion theory group is set up with Teller as head.

February, 1944 -

  • With the concrete building to house it complete, construction begins on the first reactor at Hanford, the B pile.
  • The Los Alamos Governing Board reevaluates deuterium fusion research and determines that tritium would be necessary to make an explosive reaction. Priority of fusion bomb work is further downgraded.

February 16, 1944 - Kistiakowsky becomes full-time Los Alamos staff member, replacing Neddermeyer as leader of implosion research.

March, 1944 - Segre has improved his spontaneous fission estimates in cyclotron plutonium (essentially pure Pu-239) to 11 fissions/kg-sec, this is still acceptable for gun assembly, but greatly narrows the margin of security.

March 3, 1944 - Drop tests of dummy atomic bombs begin from specially modified B-29s.

April, 1944 - IBM calculating equipment arrives at Los Alamos and is put to work on implosion research.

  • James Tuck suggests idea of using explosive lenses to create spherical converging implosion waves.
  • Monsanto begins delivering polonium for initiator research. The rate is initially 2.5 curies/month.
  • On April 5 the first sample of reactor produced plutonium arrives from Oak Ridge. Segre immediately begins monitoring its spontaneous fission rate. By April 15 he makes a preliminary estimate of a spontaneous fission rate of over 50 fissions/kg-sec (due to Pu-240 contamination), far too high for gun assembly. The report is kept quiet due to limited statistics, and observations continue.

May, 1944 -

  • Los Alamos staff exceeds 1200 employees.
  • Six months after the start of accelerated implosion research, little progress towards successful implosion has been made. Inadequate diagnostic equipment prevent accurate measurement of implosion process, no scheme to avoid asymmetry has yet shown promise. The current approach is to use many simultaneous detonation points over the surface of a sphere, and try different methods of inert spacers or gaps to suppress the shaped charge-like jets that form when detonation waves from adjacent initiation points merge. Spalling (the ejection of fragments) from the interior surface of the hollow core is a serious problem, as is simply getting precise simultaneous detonation.
  • Teller is removed as head of the implosion theory group, and also from fission weapon research entirely, due to conflicts with Bethe and his increasing obsession with the idea of the Super (hydrogen bomb).
  • Two British scientists join Los Alamos who have important impacts on the implosion program. Geoffrey Taylor (arrived May 24) points out implosion instability problems (especially the Rayleigh-Taylor instability), which ultimately leads to a very conservative design to minimize possible instability. James Tuck brings the idea of explosives lenses for detonation wave shaping (two-D lenses for plane wave generation originally proposed by M. J. Poole in England, 1942), but suggests developing 3-D lenses to create a spherical implosion.

May 9, 1944 - The 50 milliWatt Water Boiler reactor goes critical at Los Alamos. Holding 565 g of U-235 (in the form of 14.7% enriched uranyl sulfate), dissolved in a 12" sphere of water, this is the world's first reactor to use enriched uranium, and the first critical assembly constructed at Los Alamos.

May 28, 1944 - First test of the exploding wire detonator, used to achieve precise, reliable simultaneous detonation for implosion.

June, 1944 -

  • Oppenheimer replaces Neddermeyer with Kistiakowsky as director of implosion research.
  • Bethe and Peierls work on developing explosive lens concept.
  • Von Neumann provides design breakthrough for the slow component for focusing.

June 3, 1944 - After visiting the uranium enrichment pilot plan at the Naval research Laboratory, a team of Manhattan Project experts recommends that a thermal diffusion plant be built to feed enriched material to the electromagnetic enrichment plant at Oak Ridge.

June 18, 1944 -

  • Groves contracts to have S-50, a liquid thermal diffusion uranium enrichment plant, built at Oak Ridge in no more than three months.

July, 1944 -

  • Experiments with explosive lens designs begin by mid-month when 2-D models are fired.
  • The design for the gun gadget neutron initiator is completed.

July 4, 1944 - Oppenheimer reveals Segre's spontaneous fission measurements to the Los Alamos staff. The neutron emission for reactor-produced plutonium is too high for gun assembly to work. The measured rate is 50 fissions/kg-sec, the fission rate in Hanford plutonium is expected to be over 100 times higher still.

The discovery of the high spontaneous fission rate of reactor-produced plutonium was a turning point for Los Alamos, the Manhattan Project, and eventually for the practice of large scale science after the war. The planned plutonium gun had to be abandoned, and Oppenheimer was forced to make implosion research a top priority, using all available resources to attack it. A complete reorganization of Los Alamos Laboratory is required. With just 12 months to go before expected weapon delivery a new fundamental technology, explosive wave shaping, has to be invented, made reliable, and a enormous array of engineering problems had to be solved. During this crisis the many foundations for post-war science were laid. Scientist-administrators (as opposed to academic or research scientists) came to the forefront for running large scale research efforts. Automated numerical techniques (as opposed to manual analytical ones) were applied to solve important scientific problems, not just engineering applications. The dispersal of key individuals after the end of the war later carried these insights, as well as the earlier organizational principles developed at Los Alamos throughout American academia and industry.

July 1, 1944 - The Manhattan Project is granted the highest project-wide procurement priority (AA-1).

July 20, 1944 - The Los Alamos Administrative Board decides on a reorganization plan to direct the laboratory's full resources on implosion. Instead of being organized around scientific and engineering areas of expertise, all work is organized around whether it applies to implosion, or the uranium gun weapon, with the former receiving most of the resources. The reorganization is completed in less than two weeks.

August, 1944 -

  • The Air Force begins modifying 17 B-29s for combat delivery of atomic weapons at the Glenn L. Martin plant in Omaha.
  • Parsons assesses February 1945 as the earliest an implosion lens system can be ready for full scale test "with extremely good breaks", and most likely late 1945.
  • A. Francis Birch takes over the uranium gun project.

September, 1944 -

  • Air Force Lt. Col. Paul Tibbets begins organizing the 509th Composite Group, which will deliver atomic bombs in combat, at Wendover Field, Utah.
  • At this point K-25 is half built, but no usable diffusion barriers have been produced. The Y-12 plant is operating at only 0.05% efficiency. The total production of highly enriched uranium to date is a few grams.

    Now, less than one year before the eventual use of atomic weapons, the prospects for developing atomic weapons in time to assist the war effort look grim despite enormous expenditures. The only workable bomb design at hand, the gun-type weapon, requires U-235 which has no practical production methods available. Plutonium production has not yet begun, but the production techniques appear to have a high probability of success. However plausible approaches to building a plutonium bomb do not exist.

    A workable theory of explosive lenses does not exist (and is not solved before the end of the war), so trial and error techniques must be used for development. Unfortunately, observing implosions is extremely difficult and simply obtaining diagnostic data is a major barrier to success. Manufacturing test lenses is a serious problem. The explosives are difficult materials to work with and made delicate castings, mold making was a slowly developing art, and the lenses required very good quality control. During the last year of the project over 20,000 test lenses were actually used, many times this number were made and rejected. Developing a simultaneous initiation system is also a problem, as is supplying good detonators in sufficient numbers to support the test program. In light of these problems, research also continued on the non-lensed implosion approach.

    During the fall Robert Christy suggests the "Christy gadget", the use of a solid core that is raised to supercriticality solely by compressing the metal to twice normal density. This conservative implosion design avoids instability and spalling problems, but the period of maximum compression is brief and requires a "modulated initiator" (a neutron generator that emits a burst at a precise moment). Earlier shell designs could have relied on spontaneous fission and still achieved reasonable efficiency.

September 16, 1944 - S-50 enrichment plant begins partial operation at Oak Ridge, but leaks prevent substantial output.

September 22, 1944 - The first RaLa implosion test shot is made. This diagnostic technique used 100 curies of radiolanthanum produced by the X-Reactor at Oak Ridge to provide an intense gamma source for making observations of implosion (essentially an internal x-ray generator). This is the largest radioisotope source ever assembled in the world up to this time.

September 26, 1944 - Loading uranium into the first full scale plutonium reactor, the B pile, at Hanford is completed. This reactor contains 200 tons of uranium metal, 1200 tons of graphite, and is cooled by 5 m^3 of water/sec. It designed to operate at 250 megawatts, producing some 6 kg of plutonium a month. Fermi supervises reactor start-up.

September 27-30, 1944 - After several hours of operation at 100 megawatts, the B pile inexplicably shuts down, then starts up again by itself the next day. Within a few days this is determined to be due to poisoning by the highly efficient neutron absorber Xenon-135, a radioactive fission product. The reactor must be modified to add extra reactivity to overcome this effect before production can begin.

October 12, 1944 - The first B-29s arrive in the Mariana Islands to begin bombing Japan. Japan has so far remained free from air attacks (except for the symbolic Doolittle raid in 1942).

October 27, 1944 - Oppenheimer approves plans for a bomb test in the Jornada del Muerto valley at the Alamagordo Bombing Range. Groves approves 5 days later, provided that the test be conducted in Jumbo.

November, 1944 - Y-12 output has reached 40 grams of highly enriched uranium a day.

November 24, 1944 - The first B-29 raid on Japan begins. 100 planes are launched, only 16 bombs hit the target factory.

December, 1944 -

  • Y-12 output climbs to 90 grams of highly enriched uranium a day.
  • Work begins on an implosion initiator for the solid core bomb, it is not clear at this point if one can be made.

Mid-December, 1944 - First successful explosive lens tests conducted at Los Alamos, establishing the feasibility of making an implosion bomb.

December 17, 1944 - The D pile goes critical with sufficient reactivity to overcome fission product poisoning effects. Large scale plutonium production begins.

December 22, 1944 - First Fat Man bomb assembly is completed as production gets underway. Explosive lenses and nuclear material are not yet available, the bomb assemblies are used for airdrop and ground handling practice.

December 26, 1944 - Processing of irradiated uranium slugs to separate plutonium begins at Hanford.

December 28, 1944 - The modified B pile is restarted.


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